Conventionally, construction machines used in highlands with low atmospheric pressure suffer frequent engine stalls. This is because an engine output decreases as an air intake amount decreases, and as a result, an output torque of the engine is overwhelmed by an absorption torque of a hydraulic pump. Thus, a known construction machine controls a discharge amount (swash plate angle) of the hydraulic pump to reduce the absorption torque. The construction machine controls the swash plate angle of the hydraulic pump in such a manner that the actual engine speed and the target engine speed of the engine match to prevent the engine from stalling as described in Patent Literature 1 for example.
Droop control is performed for the engine of the construction machine described in Patent Literature 1, so that engine speed hunting of the engine due to abrupt fluctuation of the engine speed of the engine is prevented when the swash plate angle of the hydraulic pump is being controlled. More specifically, in the construction machine, the droop control is performed so that a required shaft torque is output with the engine speed of the engine controlled based on a predetermined change amount. Thus, in the construction machine, the engine speed of the engine is controlled so that the required shaft torque is output when a load changes while the construction machine is travelling. All things considered, there has been a problem in that a travelling speed of the construction machine can change in accordance with a road surface condition.